In the manufacture of steel plate, a poured and solidified ingot first undergoes an initial rolling operation in a rolling mill which brings it down to a thickness in the order of two inches to four inches. The slab thus formed can subsequently be rolled into steel plate with a thickness in the region of one-quarter inch to one and one-quarter inch. Almost invariably, the slab is hot-rolled, which requires that it be re-heated up to a temperature of about 1,200.degree.C in a suitable furnace, and then passed through the plate mill.
Depending upon the length of time elapsing between the removal of the slab from the re-heating furnace and the point at which ultrasonic testing is to be carried out, the surface temperature of the plate area to be tested may be anywhere from ambient to about 700.degree.C.
The basic process by which steel plate is tested for internal flaws such as laminations resulting from the rolling process involves the generation of an ultrasonic signal (longitudinal wave-propagation), together with some means for transmitting the ultrasonic signal into the steel plate without high losses. The surfacee of the steel plate act as reflection surfaces for the propagated waves, particularly the surface remote from the generation point, and by timing the lapse between the generation of the signal and the detection of the echo it is possible to distinguish between a plate area which is sound and without flaws, and one which, for example, contains a lamination or other discontinuity at a central point between its two surfaces. Any such lamination would act as a reflection surface, and the time-lapse between generation and echo detection would decrease. It is possible to display the two signals on an oscilloscope utilizing the standard synchronization techniques, so that visual identification of laminations and other flaws can be carried out.
In order to be able to transmit the ultrasonic signal directly into the material of the steel plate, a good coupling must take place, and this is ordinarily accomplished by establishing a column or layer of water between the ultrasonic signal generating surface and the nearer surface of the metal plate. Where excessive plate temperatures are encountered, the water between the ultrasonic generating surface and the plate surface can contribute to cooling to some extent, but it is important to understand that with inadequate or unstable water flow, a hot plate surface may cause boiling or bubbling, or may in some other way disrupt the coupling qualities of the liquid.